U.S. patent number 4,865,924 [Application Number 07/183,952] was granted by the patent office on 1989-09-12 for magnetic recording medium.
This patent grant is currently assigned to Fuji Photo Film Co., Ltd.. Invention is credited to Chiaki Mizuno, Hiroshi Ogawa, Toshio Ono, Shinji Saito.
United States Patent |
4,865,924 |
Saito , et al. |
September 12, 1989 |
Magnetic recording medium
Abstract
A magnetic recording medium comprises a nonmagnetic support, a
first magnetic recording layer and a second magnetic recording
layer, superposed in order, wherein each of binders contained in
said first and second magnetic recording layers comprises a
polymer, and the polymer contained in the second magnetic recording
layer has a polymerization degree of not less than 250, said
polymerization degree being more than that of the polymer contained
in the first magnetic recording layer by at least as much as
20.
Inventors: |
Saito; Shinji (Kanagawa,
JP), Mizuno; Chiaki (Kanagawa, JP), Ono;
Toshio (Kanagawa, JP), Ogawa; Hiroshi (Kanagawa,
JP) |
Assignee: |
Fuji Photo Film Co., Ltd.
(Kanagawa, JP)
|
Family
ID: |
14174849 |
Appl.
No.: |
07/183,952 |
Filed: |
April 20, 1988 |
Foreign Application Priority Data
|
|
|
|
|
Apr 20, 1987 [JP] |
|
|
62-96806 |
|
Current U.S.
Class: |
428/839.4;
428/329; 428/900; G9B/5.247; G9B/5.246 |
Current CPC
Class: |
G11B
5/7022 (20130101); G11B 5/7023 (20130101); Y10S
428/90 (20130101); Y10T 428/257 (20150115) |
Current International
Class: |
G11B
5/702 (20060101); G11B 005/64 () |
Field of
Search: |
;428/329,695,694,900 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Thibodeau; Paul J.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Claims
We claim:
1. A magnetic recording medium comprising a non-magnetic support, a
first magnetic recording layer and a second magnetic recording
layer, superposed in order, wherein each of the binders contained
in said first and second magnetic recording layers comprises a
combination of a vinyl chloride copolymer and another polymer in a
ratio in the range of 9:1 to 5:5, by weight, and the copolymer
contained in the first magnetic recording layer has a
polymerization degree of not more than 500 and the copolymer
contained in the second magnetic recording layer has a
polymerization degree of not less than 250, said polymerization
degree of the copolymer contained in the second magnetic recording
layer being more than that of the copolymer contained in the first
magnetic recording layer by at least as much as 20.
2. The magnetic recording medium as claimed in claim 1, wherein the
polymerization degree of the polymer contained in the second
magnetic recording layer is more than that of the polymer contained
in the first magnetic recording layer by at least as much as
50.
3. The magnetic recording medium as claimed in claim 1, wherein the
polymerization degree of the polymer contained in the second
magnetic recording layer is more than that of the polymer contained
in the first magnetic recording layer by at least as much as
100.
4. The magnetic recording medium as claimed in claim 1, wherein
another polymer employed in combination with the vinyl chloride
copolymer is a resin selected from the group consisting of
nitrocellulose resin, acrylic resin, polyvinyl acetal resin,
polyvinyl butyral resin, epoxy resin, phenoxy resin and
polyurethane resin.
5. The magnetic recording medium as claimed in claim 1, wherein
another polymer employed in combination with the vinyl chloride
copolymer is a polyester polyurethane resin.
6. The magnetic recording medium as claimed in claim 1, wherein the
vinyl chloride copolymer is a resin selected from the group
consisting of a vinyl chloride/vinyl acetate copolymer, a vinyl
chloride/vinyl acetate/vinyl alcohol copolymer, a vinyl
chloride/vinyl acetate/acrylic acid copolymer, a vinyl
chloride/vinylidene chloride copolymer, and a vinyl
chloride/acrylonitrile copolymer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic recording medium
comprising a nonmagnetic support and a magnetic recording layer,
and more particularly to a magnetic recording medium comprising a
nonmagnetic support and a magnetic recording layer consisting of at
least two layers.
2. Description of Prior Art
A magnetic recording medium has been widely used as a tape for
recording music (i.e., an audio tape), a video tape and a floppy
disc. The magnetic recording medium basically comprises a
nonmagnetic support and a magnetic recording layer provided on the
support, and the magnetic recording layer comprises a binder and a
ferromagnetic powder dispersed therein.
The magnetic recording medium is desired to show high levels in
various properties such as electromagnetic conversion
characteristics, running endurance and running property.
Particularly, the recording medium is desired to show excellent
electromagnetic conversion characteristics. For example, an audio
tape is required to have high reproducibility of original sounds,
and a video tape is required to have high reproducibility of
original images. Such high electromagnetic conversion
characteristics is desired particularly in the case of an 8 mm
type-video tape recorder.
It is known that the electromagnetic conversion characteristics of
the magnetic recording medium using a ferromagnetic powder greatly
varies depending upon dispersibility (or dispersed condition) of
the ferromagnetic powder in the magnetic recording layer. In more
detail, even if a ferromagnetic powder having excellent magnetic
property is used for the purpose of enhancing the electromagnetic
conversion characteristics of the resulting medium, the excellent
magnetic property of the ferromagnetic powder is not reflected on
the enhancement of the electromagnetic conversion characteristics
when the ferromagnetic powder is poorly dispersed in the recording
layer.
For improving the dispersibility of the ferromagnetic powder in the
magnetic recording layer, there has been utilized a method of
kneading or dispersing the ferromagnetic powder for a long period
of time in the preparation of a magnetic paint for forming a
magnetic recording layer. However, such long-time kneading or
dispersing causes deterioration of the magnetic property of the
ferromagnetic powder, and hence a measure of incorporating a polar
group into a resin component used in a binder of a recording layer
is recently utilized so as to make the binder show a favorable
affinity for the ferromagnetic powder.
For example, Japanese Patent Provisional Publication No.
59(1984)-5424 discloses a magnetic recording medium in which a
resin component having a specific polar group such as a metal
sulfonate group is employed in an amount of not less than 50 wt.%
as a binder of the recording layer using a ferromagnetic metal
powder. By employing such resin having a specific polar group as a
resin component of the binder, the ferromagnetic metal powder can
be well dispersed in the magnetic recording layer, whereby the
resulting recording medium can be highly improved in the
electromagnetic conversion characteristics.
With respect to the molecular weight of a resin used in a binder,
even for the above-mentioned resins having a specific polar group,
it is known that a resin of low-molecular weight can better
disperse the ferromagnetic metal powder therein and thereby improve
the electromagnetic conversion characteristics of the resulting
medium, as compared with other resins of high-molecular weight.
It is possible to prepare a magnetic recording medium improved in
the electromagnetic conversion characteristics to a certain level
as described above. However, further improvement of magnetic
recording medium in both of the electromagnetic conversion
characteristics and the running endurance is desired so far.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a magnetic
recording medium such as an audio tape or a video tape which is
improved in both of electromagnetic conversion characteristics and
running endurance.
There is provided by the present invention a magnetic recording
medium comprising a nonmagnetic support, a first magnetic recording
layer and a second magnetic recording layer, superposed in order,
wherein each of binders contained in said first and second magnetic
recording layers comprises a polymer, and the polymer contained in
the second magnetic recording layer has a polymerization degree of
not less than 250, said polymerization degree being more than that
of the polymer contained in the first magnetic recording layer by
at least as much as 20.
As described above, the magnetic recording medium of the invention
has at least two magnetic recording layers (i.e. an upper magnetic
recording layer and a lower magnetic recording layer). The lower
magnetic recording layer contains a binder comprising a polymer
having a low polymerization degree and the upper magnetic recording
layer contains a binder comprising a polymer having a high
polymerization degree. The magnetic recording medium having the
above-described constitution is excellent in the electromagnetic
conversion characteristics, particularly in the low-frequency
range, as well as the running endurance, and hence the recording
medium is satisfactory in practical use.
The above-mentioned various properties can be much more enhanced by
employing a vinyl chloride copolymer as the polymer according to
the invention.
DETAILED DESCRIPTION OF THE INVENTION
The magnetic recording medium of the present invention basically
comprises a nonmagnetic support and a magnetic recording layer
consisting two layers (i.e., a first magnetic recording layer and a
second magnetic recording layer) provided on the support. Each of
the two magnetic recording layers comprises a binder and a
ferromagnetic powder dispersed therein.
As the nonmagnetic support employable in the invention, there can
be mentioned films or sheets of synthetic resins such as polyester
resins (e.g., polyethylene terephthalate (PET) and polyethylene
naphthalate), polyolefin resins (e.g., polypropylene), cellulose
derivatives (e.g., cellulose diacetate and cellulose triacetate),
vinyl resns (e.g., polyvinyl chloride and polyvinylidene chloride),
polycarbonate, polyamide, polyamideimide, polyimide; metallic foils
such as aluminum foil and stainless steel foil; papers; and ceramic
sheets.
The thickness of the nonmagnetic support is preferably in the range
of 10 to 55 .mu.m.
The binder to be contained in the first magnetic recording layer
(i.e., lower magnetic recording layer) is required to comprise a
polymer having a polymerization degree of not less than 250, and
the binder to be contained in the second magnetic recording layer
(i.e., upper magnetic recording layer) is required to comprise a
polymer having a polymerization degree more than that of the
polymer contained in the first magnetic recording layer by at least
as much as 20. The polymer contained in the second magnetic
recording layer preferably has a polymerization degree more than
that of the polymer contained in the first magnetic recording layer
by at least as much as 50, more preferably at least as much as
100.
The polymer having a low polymerization degree which is contained
in the lower magnetic recording layer can improve the
dispersibility of the ferromagnetic powder in the recording layer
to increase a filling ratio of the ferromagnetic powder in the
recording layer as well as a squareness ratio thereof. Moreover,
the employment of the polymer having a low polymerization degree
for the lower recording layer makes it possible to enhance molding
property of the resulting sheet in a supercalendering treatment,
whereby satisfactory surface condition can be given to the
recording medium. When the polymerization degree of the polymer to
be contained in the lower recording layer is too high, the
ferromagnetic powder is poorly dispersed in the recording layer, so
that the polymerization degree thereof is preferably not more than
500.
The polymer having a high polymerization degree which is contained
in the second magnetic recording layer is thought to contribute the
running endurance and suitability for practical use of the
resulting recording medium, and the polymerization degree thereof
is preferably not less than 300.
Examples of the polymers suitably employed for the formation of
each magnetic recording layer include vinyl chloride copolymers
such as vinyl chloride/vinyl acetate copolymer, vinyl
chloride/vinyl acetate/vinyl alcohol copolymer, vinyl
chloride/vinyl acetate/acrylic acid copolymer, vinyl
chloride/vinylidene chloride copolymer, vinyl
chloride/acrylonitrile copolymer and ethylene/vinyl acetate
copolymer. Preferred are vinyl chloride copolymers having a polar
group such as a hydroxyl group, a carboxyl group, an epoxy group, a
metal sulfonate group, a phosphoric acid group and a phosphoric
acid ester group.
As the binder component, other resin component can be also employed
in addition to the above-mentioned polymers. Examples of the resin
components include cellulose derivatives such as a nitrocellulose
resin, acrylic resins, polyvinyl acetal resins, polyvinyl butyral
resins, epoxy resins, phenoxy, resins, and polyurethane resins
(e.g., a polyester polurethane resin, a polyurethane resin
incorporated with a polar group such as --SO.sub.3 Na or --SO.sub.2
Na and a polycarbonate polyurethane resin). These resins can be
employed singly or in combination.
When a curing agent is used as a component for forming a magnetic
recording layer, a polyisocyanate compound is generally employed as
the curing agent. As the polyisocyanate compound, there can be
mentioned those conventionally employed. Examples of the
polyisocyanate compounds include reaction products of tolylene
diisocyanate and 1 mole of trimethylolpropane (e.g., Desmodule
L-75, trade name, available from Bayer AG), reaction products of 3
moles of diisocyanate such as xylene diisocyanate or hexamethylene
diisocyanate and 1 mole of trimethylolpropane, burette adducts of 3
moles of hexamethylene diisocyanate, isocyanurate compounds of 5
moles of tolylene diisocyanate, isocyanurate adduct compounds of 3
moles of tolylene diisocyanate and 2 moles of hexamethylene
diisocyanate, isophorone diisocyanurate, and polymers of
diphenylmethane diisocyanate.
In the case of the subjecting the resulting magnetic recording
layer to a curing treatment by irradiating with electron beam, a
compound having a reactive double bond such as urethane acrylate
can be employed.
In the invention, it is preferred to employ a combination of a
resin having high hardness such as a vinyl chloride copolymer and a
resin having a low hardness such as a polyurethane resin as a resin
component of the binder. In the case of using a resin having a high
hardness such as a vinyl chloride copolymer in combination with a
resin having a low hardness such as a polyurethane resin, the ratio
between the former and the latter is generally in the range of 9 :
1 to 5 : 5, preferably 9 : 1 to 6 : 4, by weight.
In the case of using a ferromagnetic powder of low hardness such as
a ferromagnetic metal powder, the binder is generally used in a
larger amount than the case of using a ferromagnetic powder of high
hardness such as .gamma.-Fe.sub.2 O.sub.3, and in this case, the
amount of the resin having a low hardness such as a polyurethane
resin is generally increased. The increase of the resin having a
low hardness such as a polyurethane resin is apt to soften the
binder, so that the amount of the curing agent such as a
polyisocyanate compound is generally increased to adjust the
hardness of the binder.
When the polyurethane resin is used as a resin component and the
polyisocyanate compound is used as a curing agent, the ratio
between the polyurethane resin and the polyisocyanate compound is
generally in the range of 1 : 0.8 to 1 : 2 (polyurethane resin :
polyisocyanate compound, by weight), preferably in the range of 1 :
1 to 1 : 1.5. By setting the ratio therebetween in the
above-defined range, the binder is effectively prevented from
softening caused by the employment of polyurethane resin in a large
amount, even in the case of using a ferromagnetic metal powder
having a low hardness.
The total amount of the resin component and the curing agent is
generally in the range of 5 to 40 by weight, preferably 10 to 20 by
weight, per 100 parts by weight of the ferromagnetic powder.
Examples of the ferromagnetic powder employable in the invention
include a metal oxide type-ferromagnetic powder such as
.gamma.-Fe.sub.2 O.sub.3, a modified metal oxide type-ferromagnetic
powder such as .gamma.-Fe.sub.2 0.sub.3 containing cobalt, and a
ferromagnetic metal powder containing iron, cobalt, nickel,
etc.
The ferromagnetic metal powder employable in the invention
preferably contains iron, cobalt or nickel and has a specific
surface area of not less than 42 m.sup.2 /g, more preferably not
less than 45 m.sup.2 /g.
As a typical ferromagnetic metal powder, there can be mentioned a
ferromagnetic alloy powder containing a metal component of at least
75 wt.% in which at least 80 wt.% of the metal component comprises
at least one ferromagnetic metal or metal alloy (e.g., Fe, Co., Ni,
Fe-Co, Fe-Ni, Co-Ni and Co-Ni-Fe) and the remaining metal
component, if present, comprises other atom(s) (e.g., Si, S, Sc,
Ti, Cr, Mn, Cu, Zn, Y, Mo, Rh, Pd, Ag, W, Sn, Sb, B, Te, Ba, Ta,
Re, P, Au, Hg, Bi, La, Ce, Pr, Nd, Pd, Zn). The ferromagnetic metal
powder may contain a small amount hydroxide or oxide.
Methods of the preparation of a ferromagnetic powder are already
known, and the ferromagnetic powder employed in the invention can
be prepared according to the known methods.
There is no specific limitation on the shape of the ferromagnetic
powder employable in the invention, but normally used is a
ferromagnetic powder in a needle shape, grain shape, dice shape,
rice shape or plate shape. Preferred is a ferromagnetic metal
powder in a needle shape.
A process for the preparation of a magnetic recording medium
according to the present invention is described hereinafter.
In the first place, the above-mentioned resin component, curing
agent and ferromagnetic powder for the first magnetic recording
layer are kneaded with a solvent which is conventionally used for
the preparation of a magnetic paint such as methyl ethyl ketone,
dioxane, cyclohexane and ethyl acetate, to prepare a magnetic paint
(dispersion) for the formation of a first magnetic recording layer.
The kneading can be carried out according to a known method. A
magnetic paint for the formation of a second magnetic recording
layer can be prepared by the same manner as described above using
components for the second magnetic recording layer.
The magnetic paint for the formation of each recording layer may
contain a variety of additives or fillers such as an abrasive
(e.g., .alpha.-Al.sub.2 O.sub.3 and Cr.sub.2 O.sub.3), an
antistatic agent (e.g., carbon black), a lubricant (e.g., fatty
acid, fatty acid ester and silicon oil) and a dispersing agent
other than the above-mentioned components.
In the second place, the magnetic paint for the formation of a
first magnetic recording layer is coated over the nonmagnetic
support, and on the coated layer of the magnetic paint is then
coated the magnetic paint for the formation of a second magnetic
recording layer.
The coating procedure can be carried out by a conventional coating
method such as a method of using a reverse roll.
Each magnetic paint for the formation of the first and second
magnetic recording layers is coated in such a manner that the total
thickness of the first magnetic recording layer and the second
magnetic recording layer would be in the range of 0.5 to 10
.mu.m.
In the magnetic recording medium of the invention, a back layer (or
backing layer) may be provided on a surface of the nonmagnetic
support where a magnetic recording layer is not coated. The back
layer is generally formed on the support by coating a solution
containing a particulate component such as an abrasive or an
antistatic agent and a binder dispersed in an organic solvent over
the surface of the support not facing the magnetic recording
layer.
An adhesive layer may be also provided between the nonmagnetic
support and the magnetic recording layer or between the nonmagnetic
support and the back layer.
The coated layers of the magnetic paints for the formation of first
and second magnetic recording layers are generally subjected to
orienting the ferromagnetic powder contained in the coated layers,
that is, a magnetic orientation, and then subjected to drying.
After the drying, the resulting laminated sheet is subjected to a
surface smoothing process using for example a supercalender roll.
Through the surface smoothing process, voids having been produced
by removal of the solvent in the drying procedure disappear to
increase a filling ratio of the ferromagnetic powder in the
recording layer, whereby the resulting magnetic recording medium is
improved in the electromagnetic conversion characteristics.
Subsequently, the laminated sheet is cut or slit into a desired
shape using a known cutting or slitting device such as a slitter
under conventional conditions.
The examples and the comparison examples of the present invention
are given below. In the following examples, the expression
"part(s)" means "part(s) by weight", unless otherwise
specified.
EXAMPLE 1
______________________________________ Composition of Magnetic
Paint for First Magnetic Recording Layer Co-containing
.gamma.-Fe.sub.2 O.sub.3 100 parts (Hc: 560 Oe, s: 75 emu/g,
specific surface area: 30 m.sup.2 /g) Vinyl chloride/vinyl
acetate/vinyl alcohol copolymer 15 parts (copolymerization ratio:
91:3:6, polymerization degree: 250) Polyester polyurethane resin 5
parts (Crisbon 7209, available from Dainippon Ink & Chemicals
Inc.) Lauric acid 3 parts Butyl acetate 300 parts Composition of
Magnetic Paint for Second Magnetic Recording Layer Co-containing
.gamma.-Fe.sub.2 O.sub.3 100 parts (Hc: 700 Oe, s: 73 emu/g,
specific surface area: 40 m.sup.2 /g) Vinyl chloride/vinyl
acetate/vinyl alcohol copolymer 15 parts (copolymerization ratio:
91:3:6, polymerization degree: 300) Polyester polyurethane resin 5
parts (Crisbon 7209, available from Dainippon Ink & Chemicals
Inc.) Lauric acid 3 parts .alpha.-Al.sub.2 O.sub.3 1 part Butyl
acetate 300 parts ______________________________________
The components of each composition indicated above were kneaded in
a sand mill to give two kinds of dispersions. Each of the
dispersions was filtrated over a filter having a mean pore size of
1 .mu.m, to prepare a magnetic paint for the formation of a first
magnetic recording layer and a magnetic paint for the formation of
a second magnetic recording layer.
The magnetic paint for the formation of a first magnetic recording
layer is coated over a polyethylene terephthalate support
(thickness: 7 .mu.m) under running of the support at a speed of 60
m/min by means of a reverse roll, to give a coated layer of the
magnetic paint having thickness of 3.0 .mu.m (in dry state). On the
coated layer of the magnetic paint for the formation of a first
magnetic recording layer is then coated the magnetic paint for the
formation of a second magnetic recording layer by means of a
reverse roll, to give a coated layer having thickness of 1.5 .mu.m
(in dry state). The nonmagnetic support with the coated layers of
the magnetic paints was treated with an electromagnet at 3,000
gauss while the coated layers were wet, and then successively
subjected to a drying process and a supercalendering. The resulting
sheet was slit to give an audio tape having a width of 3.81 mm.
EXAMPLE 2
The procedure of Example 1 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 200, to prepare an
audio tape.
EXAMPLE 3
The procedure of Example 1 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 350 and further
varying the polymerization degree of the vinyl chloride/vinyl
acetate/vinyl alcohol copolymer used in the preparation of a
magnetic paint for the second magnetic recording layer from 300 to
400, to prepare an audio tape.
EXAMPLE 4
The procedure of Example 1 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 300 and further
varying the polymerization degree of the vinyl chloride/vinyl
acetate/vinyl alcohol copolymer used in the preparation of a
magnetic paint for the second magnetic recording layer from 300 to
400, to prepare an audio tape.
COMPARISON EXAMPLE 1
The procedure of Example 1 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 300, to prepare an
audio tape.
COMPARISON EXAMPLE 2
The procedure of Example 1 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 400, to prepare an
audio tape.
The audio tapes obtained in Examples 1 to 4 and Comparison Examples
1 and 2 were evaluated on the maximum output level at 315 Hz (MOL),
lowering of output and occurrence of stain on a magnetic head
according to the following tests.
The results of the evaluations are set forth in Table 1.
MAXIMUM OUTPUT LEVEL (MOL)
A signal of 315 Hz was recorded on the audio tape, and the recorded
signal was reproduced using a commercial tape deck (582 type-tape
deck, produced by Nakamichi Co., Ltd.) to measure its maximum
output level (MOL). The maximum output level is expressed by a
relative value based on the MOL value of a commercial audio tape
(FR-II of Fuji Photo Film Co., Ltd.) at high position being O
dB.
LOWERING OF OUTPUT
A signal for 60 minutes was recorded on the audio tape, and the
recorded signal was repeatedly reproduced at 10 times using a
commercial tape deck (582 type-tape deck, produced by Nakamichi
Co., Ltd.), to measure reproduction output in each reproduction.
Lowering of output of the audio tape was evaluated by expressing
the reproduction output measured at the last reproduction by a
relative value based on the reproduction output measured at the
first reproduction being O dB.
OCCURRENCE OF STAIN ON HEAD
A signal for 60 minutes was recorded on the audio tape, and the
recorded signal was repeatedly reproduced at 10 times using a
commercial tape deck (582 type-tape deck, produced by Nakamichi
Co., Ltd.). Thereafter, occurrence of stain on a magnetic head
equipped in the tape deck was observed through eye judgment.
TABLE 1 ______________________________________ Polymerization
Degree of Copolymer First Second MOL Stain Lowering Layer Layer
(dB) on Head of Output ______________________________________
Example 1 250 300 0.7 none -1.5 Example 2 200 300 1.0 none -1.0
Example 3 350 400 0.2 none -0.0 Example 4 300 400 0.3 none -0.0
Com. Ex. 1 300 300 0.3 observed -4.0 Com. Ex. 2 400 300 0.0
observed -5.0 ______________________________________
In Table 1, the term "Copolymer" means a vinyl chloride/vinyl
acetate/vinyl alcohol copolymer used in the preparation of a
magnetic paint for forming a first magnetic recording layer or a
second magnetic recording layer, and the terms "First Layer" and
"Second Layer" mean a first magnetic recording layer and a second
magnetic recording layer, respectively.
As is evident from the results set forth in Table 1, the audio tape
in which the first magnetic recording layer (lower recording layer)
and the second magnetic recording layer (upper recording layer) had
the same polymerization degree of the vinyl chloride/vinyl
acetate/vinyl alcohol copolymer as each other (Comparison Example
1) and the audio tape in which the polymerization degree of the
copolymer in the first magnetic recording layer was higher than
that in the second magnetic recording layer (Comparison Example 2)
were both markedly lowered in the output level and thereby
deteriorated in the running endurance, as compared with the audio
tapes of the present invention (Examples 1 to 4). Further, those
tapes of comparison examples caused staining on the magnetic head,
and showed unsatisfactory maximum output level at 315 Hz, resulting
in insufficient frequency properties in the low frequency
range.
On the other hand, each of the audio tapes of the invention
(Examples 1 to 4) in which the polymerization degree of the vinyl
chloride/vinyl acetate/vinyl alcohol copolymer in the first
magnetic recording layer was lower than that in the second magnetic
recording layer showed satisfactory results in all properties such
as the maximum output level, lowering of output and occurrence of
stain on the head, although they slightly vary depending on the
polymerization degree of the employed copolymer or the difference
between the polymerization degree of the copolymer in the first
magnetic recording layer and that in the second magnetic recording
layer.
EXAMPLE 5
______________________________________ Composition of Magnetic
Paint for First Magnetic Recording Layer Co-containing
.gamma.-Fe.sub.2 O.sub.3 100 parts (Hc: 600 Oe, s: 74 emu/g,
specific surface area: 30 m.sup.2 /g) Vinyl chloride/vinyl
acetate/vinyl alcohol copolymer 10 parts (copolymerization ratio:
87:8:5, polymerization degree: 250) Polyester polyurethane resin 5
parts (Crisbon 7209, available from Dainippon Ink & Chemicals
Inc.) Stearic acid 3 parts Butyl stearate 1 part Polyisocyanate 5
parts (Colonate L-75, available from Nippon Polyurethane Co., Ltd.)
Butyl acetate 300 parts Composition of Magnetic Paint for Second
Magnetic Recording Layer Co-containing .gamma.-Fe.sub.2 O.sub.3 100
parts (Hc: 750 Oe, s: 73 emu/g, specific surface area: 40 m.sup.2
/g) Vinyl chloride/vinyl acetate/vinyl alcohol copolymer 10 parts
(copolymerization ratio: 87:8:5, polymerization degree: 400)
Polyester polyurethane resin 5 parts (Crisbon 7209, available from
Dainippon Ink & Chemicals Inc.) Stearic acid 3 parts Butyl
stearate 1 part .alpha.-Al.sub.2 O.sub.3 1 part Polyisocyanate 5
parts (Colonate L-75, available from Nippon Polyurethane Co., Ltd.)
Butyl acetate 300 parts ______________________________________
The components of each composition indicated above excluding the
polyisocyanate were kneaded in a sand mill to prepare two kinds of
dispersions. To each of the dispersions was added polyisocyanate
and thery were mixed for 20 minutes. The obtained dispersion was
filtrated over a filter having a mean pore size of 1 .mu.m, to
prepare a magnetic paint for the formation of a first magnetic
recording layer and a magnetic paint for the formation of a second
magnetic recording layer.
The magnetic paint for the formation of a first magnetic recording
layer is coated over a polyethylene terephthalate support
(thickness: 15 .mu.m) under running of the support at a speed of 60
m/min by means of a reverse roll, to give a coated layer of the
magnetic paint having thickness of 3.0 .mu.m (in dry state). On the
coated layer of the magnetic paint for the formation of a first
magnetic recording layer is then coated the magnetic paint for the
formation of a second magnetic recording layer by means of a
reverse roll, to give a coated layer having thickness of 1.5 .mu.m
(in dry state). The nonmagnetic support with the coated layers of
the magnetic paints was treated with an electromagnet a 3,000 gauss
while the coated layers were wet, and then successively subjected
to a drying process and a supercalendering. The resulting sheet was
slit to give a video tape having a width of 1/2 inch.
EXAMPLE 6
The procedure of Example 5 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 200 and further
varying the polymerization degree of the vinyl chloride/vinyl
acetate/vinyl alcohol copolymer used in the preparation of a
magnetic paint for the second magnetic recording layer from 400 to
300, to prepare a video tape.
EXAMPLE 7
The procedure of Example 5 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 350, to prepare a
video tape.
EXAMPLE 8
The procedure of Example 5 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 300, to prepare a
video tape.
COMPARISON EXAMPLE 3
The procedure of Example 5 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 300 and further
varying the polymerization degree of the vinyl chloride/vinyl
acetate/vinyl alcohol copolymer used in the preparation of a
magnetic paint for the second magnetic recording layer from 300 to
300, to prepare a video tape.
COMPARISON EXAMPLE 4
The procedure of Example 5 was repeated except for varying the
polymerization degree of the vinyl chloride/vinyl acetate/vinyl
alcohol copolymer used in the preparation of a magnetic paint for
the first magnetic recording layer from 250 to 400 and further
varying the polymerization degree of the vinyl chloride/vinyl
acetate/vinyl alcohol copolymer used in the preparation of a
magnetic paint for the second magnetic recording layer from 400 to
300, to prepare a video tape.
The video tapes obtained in Examples 5 to 8 and Comparison Examples
3 and 4 were evaluated on video output, still life in a still mode
and occurrence of stain on a magnetic head according to the
following tests.
The results of the evaluations are set forth in Table 2.
VIDEO OUTPUT
A signal of 629 kHz was recorded on the video tape, and the
recorded signal was reproduced to measure video output. The video
output was expressed by a relative value based on a value of the
video output of a commercial VHS type-video cassette tape (Super HG
of Fuji Photo Film Co., Ltd.) being O db.
STILL LIFE
The video tape was run in a still mode using a commercial video
tape recorder of VHS type, to measure a term (i.e., still life) at
the end of which the video output was decreased by 6 dB.
OCCURRENCE OF STAIN ON HEAD
A signal for 60 minutes was recorded on the video tape using a
commercial video tape recorder of VHS type, and the recorded signal
was repeatedly reproduced at 10 times. Thereafter, occurrence of
stain on a magnetic head equipped in the tape deck was observed
through eye judgement.
TABLE 2 ______________________________________ Polymerization
Degree of Copolymer Video Still First Second Output Stain Life
Layer Layer (dB) on Head (min.)
______________________________________ Example 5 250 300 1.0 none
100 Example 6 200 300 1.2 none 100 Example 7 350 400 0.4 none 120
Example 8 300 400 0.4 none 120 Com. Ex. 3 300 300 0.4 observed 80
Com. Ex. 4 400 300 0.0 observed 60
______________________________________
In Table 1, the term "Copolymer" means a vinyl chloride/vinyl
acetate/vinyl alcohol copolymer used in the preparation of a
magnetic paint for forming a first magnetic recording layer or a
second magnetic recording layer, and the terms "First Layer" and
"Second Layer" mean a first magnetic recording layer and a second
magnetic recording layer, respectively.
As is evident from the results set forth in Table 1, the video tape
in which the first magnetic recording layer (lower recording layer)
and the second magnetic recording layer (upper recording layer) had
the same polymerization degree of the vinyl chloride/vinyl
acetate/vinyl alcohol copolymer as each other (Comparison Example
3) and the video tape in which the polymerization degree of the
copolymer in the first magnetic recording layer was higher than
that in the second magnetic recording layer (Comparison Example 4)
both had an extremely shorter still life and thereby deteriorated
in the running endurance, as compared with the video tapes of the
present invention (Examples 5 to 8). Further, those tapes of
comparison examples caused staining on the magnetic head, and
showed low video output, resulting in unsatisfactory
electromagnetic conversion characteristics.
On the other hand, each of the audio tapes of the invention
(Examples 5 to 8) in which the polymerization degree of the vinyl
chloride/vinyl acetate/vinyl alcohol copolymer in the first
magnetic recording layer (lower recording layer) was lower than
that in the second magnetic recording layer (upper recording layer)
showed satisfactory results in all properties such as video output,
occurrence of stain on the head and still life, although they
slightly vary depending on the polymerization degree of the
employed copolymer or the difference between the polymerization
degree of the copolymer in the first magnetic recording layer and
that in the second magnetic recording layer.
* * * * *